This application is based on Japanese Patent Applications No. 2001-278330 filed on Sep. 13, 2001 and No. 2002-171595 filed on Jun. 12, 2002, the disclosure of which is incorporated herein by reference.
The present invention relates to a vehicle air conditioner having a hot gas heating function using an interior heat exchanger (evaporator) as a radiator by directly introducing a gas refrigerant (hot gas) discharged from a compressor into the interior heat exchanger. The present invention relates particularly to a system in which condensed water is prevented from evaporating in the interior heat exchanger and from fogging a vehicle windshield in a heating mode.
In a conventional vehicle air conditioner, hot water (i.e., engine cooling water) is circulated in a heating heat exchanger during a heating operation in winter to heat air to be conditioned in the heating heat exchanger by using the hot water as a heat source. In this case, when the hot water temperature is low, the temperature of air to be blown into a passenger compartment is lowered and thus may be insufficient for a heating capacity.
Thus, JP-A No. H5-272817 proposes a vehicle air conditioner which has a heating function by using a hot gas heater cycle. When the hot water temperature is lower than a predetermined value as just after the start-up of an engine, gas refrigerant (or hot gas) discharged from a compressor is introduced into an interior heat exchanger (evaporator) while bypassing a condenser to release the heat from the gas refrigerant to the air to be conditioned in the interior heat exchanger to obtain an auxiliary heating function. That is, in the above conventional vehicle air conditioner, one heat exchanger disposed in an air conditioner case is selectively used as a cooler in a cooling mode and as a radiator in a heating mode.
By the way, in the vehicle air conditioner, an inside air mode may be set to prevent contaminated outside air from being introduced during the heating mode in winter. In this case, it is necessary for the evaporator to cool and dehumidify the air to be conditioned to prevent the windshield from fogging. Accordingly, until an outside air temperature drops to 0xc2x0 C., the refrigerating cycle may be used in the cooling mode.
After the refrigerating cycle is operated at an outside air temperature of about 0xc2x0 C. with the cooling mode to prevent the fogging of the windshield, the refrigerating cycle may be switched to the hot gas heater cycle (heating mode) to increase the heating capacity. Moreover, after the refrigerating cycle is operated with the cooling mode and then is stopped at once and is started with the hot gas heating cycle (heating mode).
In the above case, condensed water generated in the cooling mode of the refrigerating cycle remains on the surface of the interior heat exchanger. Thus, if the refrigerating cycle is started in the heating mode, the interior heat exchanger functions as the radiator of the gas refrigerant to rapidly increase the temperature of the interior heat exchanger. Accordingly, the condensed water on the surface of the interior heat exchanger evaporates, and air having a high humidify is blown into the passenger, so that the vehicle windshield is fogged.
Moreover, the condensed water once generated on the interior heat exchanger by the operation of the cooling mode does not easily evaporate at a low outside air temperature in winter and may remain for a long time. Thus, even not just after switching from the cooling mode to the heating mode, the vehicle windshield may be fogged by starting the heating mode of the refrigerating cycle.
Thus, the present inventors proposed, in JP-A No. 2000-219034, an invention of aiming to prevent the condensed water in the interior heat exchanger from evaporating and fogging the vehicle windshield in the heating mode in the vehicle air conditioner having the hot gas heating function.
In this related art, a physical quantity relating to the temperature of the windshield and the inside air humidity in the vicinity of the vehicle windshield are detected and it is determined based on this physical quantity whether or not the windshield is in the state of fogging. When it is determined that the vehicle windshield is in the state of fogging, the refrigerating cycle is controlled so as to suppress the temperature of the interior heat exchanger. More specifically, the temperature of the air blown out of the interior heat exchanger is controlled to suppress the evaporation of the condensed water in the interior heat exchanger to thereby prevent the vehicle windshield from fogging.
However, according to the specific experiment and study of the related art, since whether or not the vehicle windshield is in the state of fogging is indirectly determined (estimated) based on the physical quantities relating to the temperature of the windshield or the inside air humidity in the vicinity of the vehicle windshield, a temperature control not relating to the actual quantity of condensed water that is retained in the interior heat exchanger (in the specification, referred to as xe2x80x9cthe quantity of retained waterxe2x80x9d) may be performed.
That is, since the quantity of retained water on the interior heat exchanger is not directly determined in the related art as described above, even in the case where the condensed water is not retained on the interior heat exchanger, that is, even in the case where the control of preventing the fogging of the windshield is not actually required because there is no retained water, the temperature of the air blown out of the interior heat exchanger may be suppressed to uselessly limit the heating capacity of the hot gas heating mode.
In view of the above-described problems, it is an object of the present invention to prevent a vehicle windshield from fogging in a hot gas heating mode.
It is another object of the present invention to correctly determine whether or not the control of preventing the fogging of the vehicle windshield is necessary, and to effectively perform heating capacity in the hot gas heating mode while it can prevent the vehicle windshield from fogging.
According to the present invention, in an air conditioner, a refrigerant cycle system is constructed to switch a cooling refrigerant cycle where refrigerant discharged from a compressor is returned to the compressor through an exterior heat exchanger, a pressure reducing device and an interior heat exchanger, and a hot gas heater cycle where the refrigerant discharged from the compressor is directly introduced to the interior heat exchanger through a hot gas bypass passage while bypassing the exterior heat exchanger. Further, a control unit for controlling operation of the refrigerant cycle system performs a cooling mode for cooling air in the interior heat exchanger by using the cooling refrigerant cycle, and a heating mode for heating air in the interior heat exchanger by using the hot gas heater cycle. The control unit includes determining means for determining whether a quantity of retained water is in the interior heat exchanger, and control means for controlling temperature of air blown out of the interior heat exchanger to a range lower than a dew point even when air blown out of the air conditioning case is cooled by a vehicle windshield when the determining means determines that the quantity of retained water is in the interior heat exchanger in the heating mode. Accordingly, in the heating mode due to the hot gas heater cycle, it can accurately prevent the vehicle windshield from fogging. In addition, when the determining means determines that the quantity of retained water is in the interior heat exchanger, the control means controls the temperature of air blown out of the interior heat exchanger to a range lower than the dew point. On the other hand, when the determining means determines that the interior heat exchanger does not have the quantity of retained water, the temperature of air blown out of the interior heat exchanger is not restricted. Thus, heating capacity in the heating mode can be effectively improved.
Preferably, the control means controls the temperature of air blown out of the interior heat exchanger to be lower than temperature of the vehicle windshield when the determining means determines that the quantity of retained water is in the interior heat exchanger. Since air is forcibly passed through the interior heat exchanger even if condensed water evaporates in the heating mode, the relative humidity of air blown from the interior heat exchanger is generally about from 80% to 90%. Thus, even if air near the vehicle windshield is cooled by the vehicle windshield to be reduced to the same temperature as the vehicle windshield, the relative humidity of air near the vehicle windshield is increased to only from 80% to 90% that is same degree as the relative humidity of air blown from the interior heat exchanger.
Further, the control unit corrects a target temperature of air blown out of the interior heat exchanger with respect to the temperature of the vehicle windshield, based on a blowing mode of air blown out of the air conditioning case and a quantity of air blown into the air conditioning case. Accordingly, when the quantity of blown air is large and in the blowing mode where the rate of moisture evaporated and blown to the inner surface of the vehicle windshield is small, the target temperature of air blown from the interior heat exchanger is corrected to a higher temperature side, so that the heating capacity in the heating mode can be improved.
Preferably, the control means controls the temperature of air blown out of the interior heat exchanger by controlling a discharge capacity of the compressor. Specifically, the discharge capacity of the compressor can be controlled by an intermittent control of compressor operation, a variable control of the discharge capacity of the compressor, a variable control of the rotation speed of the compressor and the like.
Preferably, the control unit calculates the quantity of evaporation of condensed water in the interior heat exchanger in an air blowing mode, and the quantity of evaporation of condensed water in the air blowing mode is used as information for calculating the quantity of retained water. Therefore, the quantity of retained water can be accurately calculated.
On the other hand, the temperature of the vehicle windshield is calculated based on an outside air temperature and a temperature increase of the vehicle windshield due to air blown out of the air conditioning case. Therefore, the temperature of the vehicle windshield can be accurately calculated without using a temperature sensor only for detecting the temperature of the windshield. Further, the temperature increase is calculated based on information relating to the temperature of air blown out of the air conditioning case, and information relating to the quantity of air blown out toward the vehicle windshield among air blown out of the air conditioning case.
Preferably, the temperature of the vehicle windshield is calculated based on an outside air temperature, information relating to a temperature of air blown out toward the vehicle windshield from the air conditioning case, and information relating to a vehicle speed. Therefore, the temperature of the vehicle windshield can be more accurately calculated.